We have been investigating casposons, proposed to be novel mobile elements that may have been the precursor of bacterial and archaeal adaptive immune systems. We first examined the DNA sequences surrounding these proposed mobile elements, and discovered that many of them are enclosed within short, duplicated segments of genomic DNA of 14-15 basepairs. This was new and compelling evidence that a hallmark of DNA transposition - target site duplications - was also associated with casposons. We successfully expressed and purified the proposed transposase associated with one particular casposon, that from a deep sea vent archaeal species, Aciduliprofundum boonei. When we probed the biochemical properties of this casposase, we could show that the protein can integrate both short oligonucleotides and also a 2.8 kb excised mini-casposon construct into target DNA. Casposon integration occurs without target specificity and generates 14-15 basepair target site duplications, consistent with those found in casposon host genomes. This result serves as the basis for our ongoing studies to structurally characterize casposases bound to their DNA substrates.